There exist subregions of the primate brain that contain neurons that respond more to images of faces over other objects. These subregions are thought to support face-detection and discrimination behaviors. Although the role of these areas in telling faces from other objects is supported by direct evidence, their causal role in distinguishing faces from each other lacks direct experimental evidence. Using optogenetics, here we reveal their causal role in face-discrimination behavior and provide a mechanistic explanation for the process. This study is the first documentation of behavioral effects of optogenetic intervention in primate object-recognition behavior. The methods developed here facilitate the usage of the technical advantages of optogenetics for future studies of high-level vision.